The invention is in the field of reflection seismic exploration and relates specifically to accurately obtaining signals representing the normal interval time and the interval velocity of a seismic signal in a subsurface layer, and the thickness of that layer.
In reflection seismic exploration, a seismic signal source and a plurality of seismic signal detectors are arranged in an array which may be a row that is symmetric, i.e., with the source at the center of the row, or nonsymmetric. The source generates a seismic signal, for example, by a dynamite explosion at the bottom of a water filled hole drilled through the weathered layer, which emits energy that travels primarily as compressional waves. These compressional waves are reflected by interfaces separating layers with different acoustical impedances. The reflected seismic signals are detected by the detectors. Seismic data may be recorded on magnetic tapes in analog form, for in digital form by sampling the detector outputs and recording the samples as digital numbers. One complete record is called a seismogram and consists usually of twenty-four traces, each trace containing the output of one group of detectors in a spread of twenty-four detector arrays. Seismographic data are usually subjected to preliminary processing which compensates for the amplitude decrease with time, since the energy of the signal from the source decreases as it travels from the source to the reflective interfaces and back to the detector due to spreading of the seismic signal and attenuation. Another common preliminary compensation is for the varying thickness and seismic velocity of weathered material underlying different detectors and for variations in source and detector depths.
After such preliminary processing, the resulting seismographic data may be utilized to determine the seismic signal velocity from the characteristics of a curve plotted on a time-distance scale, where time is the time that the signal from the source takes to reach a detector after being reflected and the distance is the source-detector distance along the surface. The subject invention relates to finding a signal representing the velocity of the seismic signal, but not to the conventional way of determining the average signal velocity through several layers overlaying a reflector. Instead, the subject invention relates to finding a signal representing the seismic signal velocity within a subsurface layer which may be spaced from the surface by one or more layers of different material, because it is often desirable to know accurately the seismic signal velocity in a specific layer, for example, as an aid in identifying the material of which the layer consists, i.e., limestone, sandstone, etc. The invention is useful in subsurface formations comprising layers which may be either horizontal or dipping.